You can try "weld thru primer" but its not as "weld-thru" as you might think. Since this appears to be mainly a race car I'd go to bare metal around the weld area, weld, seam-seal top and bottom then prime and paint. If its sealed so nothing gets in then rust forming shouldn't be an issue.

You can get creative with the metal forming ... anything works to bend metal ... 2x4's, tree stumps, welding bottles, etc.